Because the frequency of crossing over is not uniform along the length of a chromosome, . ( ) the centimorgan is an unreliable unit of measurement, and is rarely used today pairs of homologous chromosomes contain small but significant differences in gene order the recombination frequency between two genes cannot be used to calculate the number of map units separating them recombination frequency cannot be used to determine the number of centimorgans separating two genes map units do not necessarily correlate to physical distances along the chromosome